Sentient creatures supposedly artificially created by Deus to create or modify other living creatures.
Appearance
The abilities allow them to change their own bodies and so can greatly vary in looks, often they choose to stick to one form though, but the form mostly would be very individual depending on the Engineer.
Soul Pearls
The only thing that stay the same in their bodies are soul pearls that contain their soul connection and keeps their consciousness' continuity. When destroyed it instantly kills the Engineer, destroying the connection to the soul.
Abilities
They have abilities to change any life that is based on DNA.
The Engineers have to have extensive knowledge in biology and medicine to perform their flesh-changing actions. At some point of experience the actions become more of an intuitive kind taking close to no cognitive effort.
They can change DNA of body cells, create stem cells of any kind, control the vegetative systems including metabolism of anyone whos body they can get in touch with allowing them them to heal or change any living creature or even create new ones from scratch.